Small particles (“particulate matter”) less than 2.5 micrometers in diameter are referred to as PM2.5. These particles are so small that they can get deep into the lungs and cause breathing problems. PM2.5 is caused by all types of combustion, including motor vehicles, power plants, residential wood burning, forest fires, agricultural burning, and some industrial processes. PM2.5 is carried by the wind, so particulate matter generated in one state can affect PM2.5 levels in downwind areas.